Orthodontic treatment is treatment in which a mechanical force (orthodontic force) is applied to the teeth for improvement of teeth alignment.
Non-Patent Document 1 provides detailed descriptions about the principle of the tooth movement.
With regard to the tooth movement in the orthodontic treatment, the periodontal ligament plays important roles as responding tissues to a mechanical force generated by an orthodontic appliance.
The periodontal ligament is fibrous tissues having a width of about 0.2 mm, the fibrous tissues being interposed between the alveolar bone and the cementum covering a surface layer of the root. The periodontal ligament is composed of various cells such as the periodontal ligament fibroblasts, osteoblasts, osteoclasts, cementoblasts, Malassez' epithelial rest cells, macrophages and vascular endothelial cells.
Under physiological conditions, the periodontal ligament is important for supporting teeth, sensory reception, supplying nutrients by the vascular plexus, and maintaining and regenerating the periodontal tissues. The periodontal ligament also has a role as a buffer zone which protects the teeth and circumjacent tissues from mechanical stimulation such as a chewing force and occlusal force.
Different types of mechanical stimulation in tension and compression forces are transmitted to the periodontal ligament and the alveolar bone of the teeth, to which an orthodontic force is applied, to cause histologically different regions in the tension and compression sides. In the tension side, the periodontal ligament stretched to expand a periodontal ligament space. Subsequently, as a result of apposition of bone promoted by the osteoblasts in a surface layer of the alveolar bone, the expanded periodontal ligament space returns to a width comparable to that under the physiological state. On the other hand, in the compression side, the root gets closer to the alveolar bone so that compression of the periodontal ligament and a narrowed periodontal ligament space are observed. Bone resorption is then promoted by osteoclasts in the compression side.
In this manner, the alveolar bone changes in shape during the tooth movement as a result of bone resorption on compression side primarily happening to the anterior alveolar bone of the moving tooth and formation of bone primarily happening to the posterior alveolar bone.
Multi-bracket appliances and orthodontic mouthpieces are currently used as orthodontic therapeutic appliances. These appliances use a return force of bent wires or deformed elastomer materials to give a continuous force to the teeth.
However, the orthodontic treatment using the continuous force requires for the orthodontic appliances to be always attached to the teeth throughout a treatment period which is several years. This means a considerable physical and psychological burden on patients.
Application of a dynamic load such as vibration instead of a continuous force is considered as one of effective methods to shorten an orthodontic treatment period. There have been researches conducted previously.
Non-Patent Document 2 (1978) describes a comparison between a case of applying a continuous force of 69.3 gf to the left upper canine teeth of German shepherds using a coil spring and a case of applying an intermittent force to the upper right canine teeth, the magnitude of the intermittent force being controlled by using a similar coil spring and a pulse generator. With regard to the intermittent force application, 17 seconds for applying a force of 66.0 gf and 3 seconds for applying no force are repeated for the first 6 days, and then 17 seconds for applying a force of 49 gf and 3 seconds for applying no force are repeated in the next 6 days. It was reported that the upper right canine, to which the intermittent force had been applied, showed a greater increase in a movement amount after 12 days.
Non-Patent Document 3 (1986) describes a comparison in Japanese macaques between the upper left lateral incisors, to which vibrational stimulation were applied for 1.5 hours per day at a maximum load of 40 gf, average load of 25 gf, load amplitude of ±15 gf and frequency of 115 Hz to 140 Hz, and the upper right lateral incisors, to which a continuous force of 40 gf were applied. Consequently, it was reported that the upper left lateral incisors had shown a greater increase in a tooth movement amount after 3 weeks.
Although these Non-Patent Documents 2 and 3 show effects on shortened orthodontic treatment under the application of vibrational stimulation, application to humans is impractical since a large vibration generator is attached for a long period of time and vibration having a large load are continuously applied.
Non-Patent Document 4 (2001) shows a comparison of a movement amount between the upper second bicuspids of the same beagle dog. One of the upper second bicuspids was subjected to a continuous force of 80 gf in addition to high-frequency vibrational stimulation at the amplitude of 100 μm and frequency of 28.069 kHz for 2 minutes once every 2 weeks whereas the other of the second bicuspids on the opposite side was subjected only to a continuous force of 80 gf. Consequently, it was reported that there had been a greater tooth movement amount if the continuous force was combined with the high-frequency vibrational stimulation than if only the continuous force is used.
Non-Patent Document 5 shows a study about viscoelastic characteristics of human periodontal tissues, and provides a model of a dynamic system composed of the teeth, periodontal ligament and alveolar bone as shown in FIG. 1.
Non-Patent Document 6 (2003) reports that suture growth of rabbits in a growth period was promoted under an application of a periodic force at a frequency of 1 Hz to the cranial suture 10 minutes a day for 12 days, the periodic force having a maximum load as a compression force of 5 N. Since the load application did not aim at moving the teeth in a fixed direction through the periodontal ligament with viscoelasticity, the load was considerably larger than a load used for moving teeth in the field of orthodontic treatment. Although the maximum load and frequency are described, there are no disclosures about a size of the vibrational load.
Likewise, Patent Document 1 (2000) reports that the suture growth was promoted under applications of periodic forces at frequencies of 0.2, 0.4, 0.6, 0.8 or 1.0 Hz, each of which was applied 10 minutes a day for 12 days to the cranial sutures of rabbits, the periodic forces having maximum loads 5 N or 2 N. Like Non-Patent Literature 6, there are no disclosures about a size of the vibrational load although the maximum load and frequency are described.
The sutures are fibrous tissues, which join bones composing the neuro-cranium and facial cranium. The sutures are known to be a major site of the cranial growth. In short, the cranial growth is induced by differentiation of the mesenchymal cells existing in the sutures into the osteoblasts through the osteoblast precursor cells, the osteoblasts adding the new bone to the suture borders.
Due to progression of the bone addition, all sutures eventually change over to the synostosis. In humans, the frontal suture closes one year after birth whereas other sutures further gradually change over to the synostosis during adulthood. In this regard, it is expected that there are differences from the periodontal ligament, which maintains homeostasis as fibrous tissue through the life without calcification.
In addition to their roles in the cranial growth, the sutures also have a role of receiving mechanical stimulation in the neuro-cranium and facial cranium. With regard to orthodontic treatment, patients during the growth period having skeletal discrepancy of the maxillofacial region are treated by applying mechanical stimulation to the sutures as a maxillary orthopedic force. It is observed that there are an increase in a suture width, acceleration of cell proliferation, increased extracellular matrix production and calcification at borders with bone as a result of applying a tension force to the sutures. This implies that bone formation is promoted by an application of the tension force. On the other hand, it is expected that growth at the sutures is inhibited by application of a compression force to the sutures. In a recent report, an application of a compression force to the sutures results in activation of osteoclasts and bone resorption which in turn leads to compression of the sutures and inhibits bone growth.
When mechanical force is applied to the teeth, a tension side and a compression side are simultaneously observed in the alveolar bone in correspondence to a stimulation direction, so that each of bone resorption and bone formation progresses in each side. On the other hand, tissue reactions in the sutures loaded by one of tension force and compression force cause one of bone formation and bone resorption around the sutures subjected to the stimulation. In this manner, there are different dynamics between the periodontal ligament and the sutures under mechanical stimulation. Therefore, the vibration parameters such as a load or application period cannot be directly applied to orthodontic treatment since mechanism of action under an application of vibration to the periodontal ligament is different from application of vibration to the sutures described in Non-Patent Documents 1 and 6.
A device configured to apply vibration to dentition when a user bites the planar device to which a vibrator is connected is known as an attempt of a practical device for applying vibration to the teeth (c.f. Patent Document 2). There is another device which applies a pulsed load to the entire dentition when a mouthpiece with an embedded metal wire is attached, the mouthpiece being connected to an extra-oral transducer (c.f. Patent Document 3). There is another device including a metal orthosis ring, which is attached to an individual tooth, and an extra-oral transducer, which is connected to the metal band, the device applying a pulsed load to an arbitrary tooth fitted with the orthosis (c.f. Patent Literature 4). However, the object of Patent Document 2 is to promote gingival circulation and alleviate pain accompanying orthodontic treatment. Patent Document 2 does not describe how to shorten an orthodontic treatment period. In addition, although a period of 10 minutes to 15 minutes is disclosed as one of the parameters of vibration application period, there are no experimental data as grounds and no specific descriptions about a size of the vibrational load. Although the objects of Patent Documents 3 and 4 are to shorten an orthodontic treatment period, like the present invention, there are no specific descriptions about vibration characteristics.
There are also devices of a bite plate type, which are used for the purpose of shortening an orthodontic treatment period, with description about their vibration characteristics (Patent Documents 5, 6 and 7).
Non-Patent Literature 5 proposes a device which applies vibrations at 0.1 Hz to 40 Hz having a maximum load of 0.1 N to 10 N on the basis of experimental data in rabbits. However, these values are based on the experimental data in which suture growth was promoted under an application of vibrational stimulation to rabbit cranium. Non-Patent Literature 5 discloses experimental data under applications of compression forces with maximum loads of 2 N and 5 N but there is no description about other vibration characteristics. In addition, there is no experimental data about tooth movement through the periodontal ligament.
Patent Documents 6 and 7 propose devices configured to apply vibrations of 0.1 N to 10 N at 0.1 Hz to 1200 Hz for 1 minute to 60 minutes. Patent Document 8 proposes a device configured to apply vibrations of 0.01 N to 3 N at 0.1 Hz to 1000 Hz. These documents reveal vibration characteristics and device structures but do not disclose any data as grounds.